Modern portable communication devices (e.g., cell phones, PDAs, etc.) comprise transmission chains configured to transmit information wirelessly using electromagnetic waves. FIG. 1a illustrates an exemplary direct conversion (homodyne) transmitter 100 that utilizes quadrature modulation to encode data onto a composite modulated output signal, that can be wirelessly transmitted by an antenna, through the use of amplitude modulation (i.e., varying the strength of the output signal) and frequency modulation (i.e., varying the frequency of the output signal).
As shown in FIG. 1a, a baseband processor 102 is configured to generate in-phase (I) and quadrature phase (Q) equivalent baseband signals having a frequency range centered around zero Hz (e.g., −8 MHz to 8 MHz). The I and Q equivalent baseband signals are output from the baseband processor 102 to respective up-conversion mixers 106a and 106b. A local oscillator 104 is configured to generate an oscillator output signal SOSC (e.g., a sin wave) at a high frequency (e.g., 10 GHz), which is provided to a quadrature divider 108 that is configured to divide the frequency of the oscillator output signal SOSC by a division factor to generate local oscillator signals, LOI(0°) and LOQ(90°), which are offset by 90°. The local oscillator signals are provided to the up-conversion mixers 106a and 106b, which modulate the I and Q equivalent baseband signals onto the local oscillator signals, thereby up-converting the frequency of the I and Q equivalent baseband signals and generating mixer output signals having a frequency equal to that of the local oscillator signals. The mixer output signals are combined by an adder 110 to form a composite modulated output signal SCOMP that is provided to one or more amplification stages 112 before being received by an antenna 114 for wireless transmission.
The one or more amplification stages 112 are configured to amplify the composite modulated output signal SCOMP, having a small amount of energy, to form a transmitter output signal ST—OUT with a larger amount of energy. However, since the oscillator output signal SOSC is at substantially the same frequency as the composite modulated output signal SCOMP, the output signal of the one or more amplification stages 112 (or any higher harmonic frequency of it) may interfere with operation of the local oscillator 104 (e.g., shift and/or modify the frequency of the oscillator output signal), which is also at a multiple of the output center frequency, due to the LO-frequency division, and is therefore susceptible to external disturbances. Such external disturbances may result in spurious emissions being output from the transmitter 100.
For example, FIG. 1b illustrates frequency spectrum diagrams corresponding to transmitter 100. Frequency spectrum diagram 116 illustrates the frequency fOSC of the oscillator output signal SOSC and the frequency fCOMP of the composite modulated output signal SCOMP (i.e., substantially equal to the frequency of the local oscillator signals, LOI(0°) and LOQ(90°)). The frequency fCOMP is equal to the frequency fOSC divided by a division factor (e.g., 2 or 4), as described above.
Referring to frequency spectrum diagram 118, the frequency fCOMP of the composite modulated output signal will have harmonic frequencies having a reduced power. The power of the harmonic frequencies decreases with each order of harmonic, but since the division factor between the fOSC and fCOMP is typically small, the harmonic frequencies may be strong enough to disturb operation of the local oscillator 104. Therefore, since the frequency fOSC is substantially equal to first harmonic frequency, the first harmonic frequency may interfere with the frequency fOSC causing remodulation (i.e., the superposition of spurious harmonic frequencies onto frequency fOSC).
Furthermore, as shown in frequency spectrum diagram 120, since the first harmonic frequency fCOMP is substantially equal to frequency fOSC, the quadrature divider 108 may perform division of both frequency fOSC and the first harmonic frequency, resulting in a heavily disturbed spectrum around frequency fCOMP and significant phase error in composite modulated output signal SCOMP that may degrade the quality of transmitter output signal ST—OUT at antenna 114.